Research Progress on Synergistic Effect between Insulation Gas Mixtures

*Su Zhao and Dengming Xiao*

### **Abstract**

Synergetic effect is a special gas discharge phenomenon among insulating gas mixtures, which has important reference value for gas selection of future gasinsulated power equipment. The research progress and investigation methods of synergistic effect and insulation characteristics of different gas mixtures at home and abroad are reviewed in this chapter. The synergistic effect between different kinds of gas mixtures including SF6 gas mixtures and some new insulation gases such as c-C4F8, CF3I, and C4F7N is presented. Combined with the results of multiple studies, it can be seen that the synergistic effect of the gas mixture has a certain relationship with the electronic transport parameters and discharge patterns. Besides, the synergistic effect of the same gas mixture may change with the change of external conditions such as gas pressure, voltage type, and electrode distance.

**Keywords:** gas discharge, synergistic effect, gas mixture, insulation, environmental friendly gas

### **1. Introduction**

With the continuous development of power industry, gas-insulated power equipment has been used more and more widely in electrical transmission system. Gas-insulated power equipment mainly includes gas-insulated transformer (GIT), gas-insulated transmission line (GIL), and gas-insulated switchgear (GIS). These gasinsulated equipment are noncombustible, nonexplosive, safe, and stable and have long maintenance period [1]. At present, SF6 gas is the most widely used gas in power equipment, which has excellent insulation performance and arc extinguishing ability. As early as the 1940s, SF6 gas has been used as insulation gas in power equipment [2]. Since the 1970s, researchers have studied the use of gas mixture including SF6 and some buffer gases such as N2, CO2, Ar, He, air, etc. to replace pure SF6 as the insulation medium [3–5]. With further study, researchers found that the insulation characteristics of gas mixtures containing SF6 gas did not increase linearly according to the mixing ratio of SF6, and the electrical strength of the mixture was higher than the weighted average of the electrical strength of the two gas components. In 1980, Wootton and Chantry first used synergism to describe this phenomenon [6]. In the study of synergistic effect, scholars have found that there are also super synergistic effect and negative synergistic effect, which are shown in **Figure 1**.

Suppose that the composition of gas mixture is gas 1 and gas 2, the breakdown voltage of gas 1 is *U*1, the breakdown voltage of gas 2 is *U*2, and *U*1 > *U*2, the

**Figure 1.** *Types of synergistic effect.*

breakdown voltage of gas mixture is *U*m, and *K* is the mixing ratio of gas 1 in the gas mixture. For super synergistic effect, *U*m > *U*1 and *U*m > *U*2 exist in some mixing ratios, as shown in curve a; for synergistic effect, *U*m > *U*1 + *U*2, as shown in curve b; for linear relationship, *U*m = *U*1 + *U*2, as shown in curve c; for negative synergistic effect, *U*m < *U*1 + *U*2, as shown in curve d in **Figure 1**.

At first, researchers studied the synergistic effect of SF6 gas mixture in order to solve the problems of high liquefaction temperature, sensitivity to electric field, and the high price of SF6 gas [7]. However, with the deepening understanding of SF6, scholars have found that SF6 is a strong greenhouse gas. It is estimated that the global annual production of SF6 gas is more than 20,000 tons and 80% of SF6 gas produced globally each year is used in the power industry. Although SF6 has many advantages, its greenhouse effect on the earth cannot be ignored. The global warming potential (GWP) value of SF6 gas is 23,900; it means that the emission of 1 kg SF6 is equivalent to the emission of 23,900 kg of CO2. What is more serious is that SF6 has a very stable chemical property, which is difficult to decompose after it spreads to the outside environment, and can exist for up to 3200 years. The environmental impact and greenhouse effect generated by SF6 will continue to accumulate [8–10].

Affected by climate change, more and more international cooperation has been carried out to reduce greenhouse gas emissions, so as to curb global climate change and maintain the sustainable development of the environment. In the Kyoto protocol of the United Nations framework convention on climate change signed in Kyoto, Japan, in 1997, SF6 has been clearly regulated as one of the six greenhouse gases and requires developed countries to freeze and reduce the total greenhouse gas emissions [11]. It means that the use of SF6 in the industrial field will be increasingly restricted and pressured. Therefore, it is an urgent task to study a new gas insulation scheme to replace SF6.

Although at present there have been many studies about alternative SF6 gas, no gas can thoroughly replace SF6 gas in the form of a single gas; they all have to be mixed with buffer gas for industrial application. The reasonable use of synergistic effect can effectively improve dielectric strength of gas mixtures and reduce the use of insulation gas, so in this paper, the research progress and methods of synergistic effect with gas mixtures are introduced; the prospect and the difficulties in the field were also discussed. This paper is expected to provide help and reference for future research on synergistic effect.

**109**

**Figure 2.**

*methods.*

*Research Progress on Synergistic Effect between Insulation Gas Mixtures*

the calculation results with the actual values and weighted values [12].

In view of the synergistic effect and insulation strength of SF6 mixture, scholars studied and analyzed it through theoretical calculation and experimental research. **Figure 2a** is a simple and intuitive calculation method proposed by Wieland et al. for the insulation strength of SF6 gas mixture, and **Figure 2b** is the comparison of

Christophorou et al. thought that the preferred gas mixture should include effective electron-attaching gas and/or electron-slowing down gas [13]. The attachment cross section of electron-attaching components should be as wide as possible, or the attachment cross section of different gases is in different energy interval, so the attachment cross section of gas mixtures is also wide. The effect of electron-slowing down gas is to slow down the free electrons, making them easier to attach and reducing secondary ionization. Based on this theory, they believed that when an electronegative gas and a gas with a large dipole moment are mixed, the synergistic effect and insulation strength of the gas mixture will be better. **Figure 3** shows the experimental results of SF6 gas mixtures with CF4, CHF3, and 1,1,1-CH3CF3 gas, and the electric dipole moments of these three gases are 0, 1.65D, and 2.32D, respectively. From the figure, it can be seen that the breakdown voltage curve of SF6-CF4 gas mixtures shows almost a linear trend, and the electric dipole moments of CF4 is 0. When it comes to SF6-CHF3 gas mixture, as the content of SF6 gas increases, the breakdown voltage of the gas mixtures does not increase in a straight line, and there is synergistic effect that occurs. The electric dipole moments of 1,1,1-CH3CF3 gas is 2.32D, which is the one with the largest electric dipole moment among the three gases; from **Figure 3c** it can be seen that the synergistic effect of this gas mixture is

Okubo et al. investigated the partial discharge (PD) and breakdown characteristics of SF6-N2 gas mixtures in order to analyze the relationship between electronegativity, additive gases, and the insulation strength [14]. They believed that the synergistic effect of gas mixture is related to the change of discharge form. **Figure 4** shows the

*Experimental results of SF6 gas mixtures. (a) Wieland calculation method, (b) Results comparison of different* 

*DOI: http://dx.doi.org/10.5772/intechopen.90705*

the most pronounced.

**2. Synergistic effect in SF6 gas mixtures**
